Microprocessor Integrates Silicon Photonics

Photonics SpectraFeb 2016
Made from silicon using conventional fabrication methods, a microprocessor that integrates multiple photonic components is said to be the first to communicate directly with other devices using light.

The device consists of two processor cores with more than 70 million transistors and 850 photonic components on a 3 × 6-mm chip. It was fabricated in a foundry that mass-produces computer chips, indicating that their design could be scaled up for commercial production.

The electronic-photonic processor chip communicates directly with other devices using light. This photo shows the chip naturally illuminated by red and green bands of light. Courtesy of Glenn J. Asakawa/University of Colorado.
According to its developers, the chip marks the next step in the evolution of fiber optic communications technology by integrating into a microprocessor the photonic interconnects, or inputs and outputs (I/O), needed to talk to other chips.

"This is a milestone. It's the first processor that can use light to communicate with the external world," said Vladimir Stojanovic, a professor at the University of California, Berkeley, who led development of the chip. "No other processor has the photonic I/O in the chip."

Researchers from the Massachusetts Institute of Technology and the University of Colorado, Boulder, also took part in the project.

The team verified the chip's functionality by using it to run various computer programs, requiring it to send and receive instructions and data to and from memory.

"This is the first time we've put a system together at such scale, and have it actually do something useful, like run a program," said Berkeley professor Krste Asanovic, who helped develop the free and open architecture, called RISC-V (reduced instruction set computer), used by the processor.

The chip demonstrated a bandwidth density of 300 Gb/s per square millimeter, about 10 to 50 times greater than packaged electrical-only microprocessors currently on the market.

A microscope image of the chip reveals its electrical and photonic features. Courtesy of Chen Sun/UC Berkeley.
The photonic I/O on the chip is also energy-efficient, using only 1.3 pJ per bit, equivalent to consuming 1.3 W of power to transmit a terabit of data per second. In the experiments, data was sent to a receiver 10 m away and back.

"The advantage with optical is that with the same amount of power, you can go a few centimeters, a few meters or a few kilometers," said Chen Sun, a recent Berkeley doctoral graduate who worked on the project. "For high-speed electrical links, 1 m is about the limit before you need repeaters to regenerate the electrical signal, and that quickly increases the amount of power needed. For an electrical signal to travel 1 km, you'd need thousands of picojoules for each bit."

Each of the key photonic I/O components — such as a ring modulator, photodetector and a vertical grating coupler — serves to control and guide the light waves on the chip.

The design had to conform to the constraints of a process originally thought to be hostile to photonic components. To enable light to move through the chip with minimal loss, for instance, the researchers used the silicon body of the transistor as a waveguide. They did this by using available masks in the fabrication process to manipulate doping, the process used to form different parts of transistors.

After getting the light onto the chip, the researchers needed to find a way to control it so it carries bits of data. They designed the silicon ring with p-n-doped junction spokes next to the silicon waveguide to enable fast, low-energy modulation of light.

Using the silicon-germanium parts of a modern transistor to build the photodetector took advantage of germanium's ability to absorb light and convert it into electricity.

The vertical grating coupler leverages existing polysilicon and silicon layers to connect the chip to the external world, directing the light in the waveguide up and off the chip.

One near-term application for this technology is to make data centers more green. According to the Natural Resources Defense Council, data centers consumed about 91 billion kWh of electricity in 2013, about 2 percent of the total electricity consumed in the United States, and the appetite for power is growing exponentially.

This research has already spun off two startups this year with applications in data centers in mind. SiFive is commercializing the RISC-V processors, while Ayar Labs is focusing on photonic interconnects.

Under its previous name, OptiBit, Ayar Labs was awarded the MIT Clean Energy Prize earlier this year. The company is getting further traction through the CITRIS Foundry startup incubator at UC Berkeley.